The invention relates to the pressure swing adsorption process (PSA) for the separation of a gaseous mixture.
In general, in the PSA process, the first phase comprises selectively adsorbing components of the feed gas under pressure to obtain a gaseous stream depleted of the adsorbable components. Following the adsorption phase, the adsorber is subjected to multistage expansion to desorb the previously adsorbed components. Then the pressure is increased to the adsorption pressure and the cycle is repeated.
In such PSA separation processes, to facilitate and/or complete regeneration of the adsorbent, a purge gas is generally passed at the lowest process pressure through the adsorber to flush out desorbed proportions from the adsorber and to lower the partial pressure of the components to be desorbed.
In PSA processes, it is also conventional to interrupt the loading of an adsorber before the adsorption front has reached the outlet end of the adsorber. This yields the advantage that the subsequent expansion of the adsorber can be conducted in part cocurrently to the adsorption direction, the adsorption front progressing further to the outlet end, but with insignificant breakthrough, if any at all, of the adsorption front. Cocurrent expansion can be conducted in one or more stages. Before substantial amounts of adsorbable components exit from the outlet end of the adsorber, the expansion is then generally switched to countercurrent flow. The expansion gases obtained during cocurrent expansion are conventionally employed for the pressurization of other, previously regenerated adsorbers, or for purging adsorbers to be regenerated. One process of this type is known, for example, from DOS 3,346,032.
This type of operation, though well-proven and widely used under practical conditions, is not entirely satisfactory insofar as the adsorbent at the outlet end of an adsorber is not used to its full capacity, i.e., it is used merely to prevent breakthrough of adsorbable components during cocurrent expansion phases.